This invention relates in general to optical data transfer systems and especially to integrated optical switches and modulators. More particularly, this invention relates to polarization-independent integrated optical switches and modulators suitable for use with fiber optical transmission lines.
In the past several years, significant progress has been made in the construction of microoptical thin-film electro-optical switches and modulators. These devices are capable of impressing broadband information on optical carriers with efficiencies heretofore not realizable. However, most of these devices suffer from large interchannel crosstalk caused by the inability to fabricate these devices within demanding tolerance requirements. An approach to alleviating the demanding tolerance requirements in channel-waveguide switches was recently demonstrated by H. W. Kogelnik and. R. V. Schmidt (See U.S. Pat. No. 4,012,113 and IEEE Journal of Quantum Electronics, Vol. QE-12, No. 7, July 1976). Their design significantly reduced interchannel crosstalk by compensating for errors in fabrication by application of voltage to multiple-section electrodes. The drawback of their design was that, in general, it was restricted to a single polarization; that is, it could only efficiently switch a single polarization at a time.
The polarization properties of integrated optical switches and modulators are of great importance in determining the usefulness of these devices in an optical data transfer system employing fiber transmission lines. In particular, these devices must perform efficient and complete switching of light, without regard to its state of polarization. This requirement arises because linearly polarized light coupled into single mode, circular fibers suffers a rapid conversion to other polarization states. Light coupled from a fiber is therefore expected to possess an unknown elliptical polarization and both TE- and TM-like modes will be excited in the integrated optical circuit. Any optical switch must act in identical fashion upon each of the constituent polarizations in order to achieve suitably low interchannel crosstalk.